High strength, long durability structural fabric/seam system

ABSTRACT

A high strength, high modulus structural fabric product and the method of manufacturing the product are disclosed. The incorporation of a specific fiber/fabric treatment coupled with a resin impregnation and coating process produces a composite material. This composite material comprises high strength and modulus fibers embedded in and linked to a matrix. The resulting fabric product is useable in the formation of seamed structures, which carry and distribute high-level loads under extreme environmental conditions.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of, and claims thebenefit of, U.S. patent application Ser. No. 10/773,125 filed on Feb. 5,2004, which application claims the benefit of U.S. ProvisionalApplication No. 60/445,940 filed Feb. 7, 2003, which applications arehereby incorporated herein by reference in their entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This work was sponsored under Department of Defense Contract No.HQ006-01-C-001. The government has certain rights in this invention.

FIELD OF THE INVENTION

The present invention relates generally to treated woven, knitted orunidirectional fabric, and more particularly to a treated woven, knittedor unidirectional fabric which is used to provide structures that canwithstand high level working loads and extreme environmental conditions.

BACKGROUND OF THE INVENTION

Woven or knitted fabrics coated with a resin such as Poly-Vinyl Chloride(PVC), polytetrafluorethylene (PTFE), urethane or other suitable resinhave been used to provide structures. The fibers or fiber bundles of thewoven fabric are coated with a resin, thereby forming a matrixsurrounding the fibers. The matrix material is typically the medium, bywhich seams in the construction, can transfer load across the joints inthe fabric. One example of such a structure is a radome, which is a domeshaped protective housing used to cover a radar antenna. A radome may besubject to a severe set of conditions such as supporting heavy loads forextended periods of time at extreme temperatures and humidity.

Traditional fabric structures apply resin-based coatings to the fabricsubstrate via ‘knife-over-roll’ or film lamination techniques. In orderto take structural advantage of a fiber structure, applied loads shouldbe able to transfer from one fiber bundle to another and the full loadcapability of the fabric should be able to be transferred across jointsin the fabric. The resin system applied to the fabric assists with loadtransfer. However, the effectivity of this load transferring ability isdirectly related to the interface between the fiber and the resin. Thisinterface is dependent both on the volume of surface contacted, thelinkage between the fiber surface and the resin, and the resinproperties. When a fabric is coated with resin, the coating is only incontact with the exposed outer surface of the fiber bundles and,effectively, the fiber/resin product (also referred to as the fabricproduct) is full of voids or air pockets within the fiber bundles. Whenan applied load encounters a void, the load cannot be transferred orcarried from fiber filament to filament. The propagation of the loadeffectively stops and a stress concentration develops that eventuallyexceeds the fabric load resistance, resulting in a failure of the fabricproduct. This effect is most pronounced at the fabric seam locationswhere the fiber bundles are not continuous across the joint.

A drawback associated with these coated woven fabrics and seams is thatwhen they are utilized to provide a structure such as a radome, theytypically do not withstand the long duration, high level working loadsand extreme environmental conditions.

One attempt to resolve the above-mentioned drawback was to increase thebase fabric load carrying capability, load transfer capability and fiberbundle load sharing capability by modifying existing fabric weavedesigns. These attempts did not produce a fabric/seam system product,which could withstand the working loads and environmental conditionsmentioned above. Another attempt to overcome the above mentioneddrawback involved development of new fabric/fiber technology. Althoughthis attempt utilized innovative fiber and fabric designs, this attemptrelied on traditional fabric coating technology that failed to achievethe required combination of properties between the fiber system and theresin matrix, resulting in premature seam failure. The net result ofboth attempts was unacceptable operation in the area of seam performanceof the structure.

SUMMARY OF THE INVENTION

A high strength, high modulus structural fabric product and the methodof manufacturing the product are presented. The incorporation of afiber/fabric treatment coupled with a resin impregnation and coatingprocess produces a composite material. This composite material compriseshigh strength and modulus fibers embedded in and coupled to a matrixsuch that the resin matrix material penetrates to the filament level ofthe fiber bundle. The resulting fabric product is useable in theformation of structures, which carry and distribute high-level loadsacross seams under extreme environmental conditions.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood from the following detaileddescription taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a diagram of the fabric product; and

FIG. 2 is a flow chart of the process utilized to produce the fabricproduct.

DETAILED DESCRIPTION OF THE INVENTION

The present invention comprises a high strength, high modulus structuralfabric product and the method of manufacturing the product. Utilizing afiber/fabric treatment and a resin impregnation and coating process afabric product useable in air-supported structures, which is able tocarry and distribute high level loads across seams under extremeenvironmental conditions and with high levels of survivability isprovided. The incorporation of a fiber/fabric treatment coupled with aresin impregnation and coating process produces a composite material.This composite material comprises high strength and modulus fibersembedded in and coupled to a matrix such that the resin matrix materialpenetrates to the filament level of the fiber bundle. In this form, boththe fibers and the matrix retain their physical and chemical identities,and further produce a combination of properties that are not achievedwith either constituent acting alone.

In general, the fibers of the fabric are the principal load carryingmembers. The fabric is combined with a surrounding resin matrix whichkeeps the fibers in a desired location and orientation. The surroundingmatrix further acts as a load transfer medium between the fibers,provides a shear load transfer medium across seams and protects thefibers from damage due to environmental conditions such as temperature,humidity, and sunlight.

The present invention utilizes a resin application technology thateffectively ‘impregnates’ a fabric substrate such that the resinsurrounds fibers and fiber bundles and infiltrates the fiber bundles tothe filament level. The result is a fabric structure that has thecombined properties of the resin for load transfer capabilities and thefiber/fabric system to carry loads.

Referring to FIG. 1, a sectional isometric view of a particularembodiment of a fabric product 1 incorporating the present invention isshown. The fabric product 1 includes resin impregnated fabric layer 60.The resin impregnated fabric layer includes a fabric made of fibers suchas Vectran®, Kevlar® or other high performance fiber which has beenimpregnated with the resin. As discussed above, the resin surrounds andinfiltrates the fiber bundles of the fabric to the filament level.

The resin impregnated layer 60 has, a resin coating layer 50 disposedacross a top surface and a resin coating layer 52 disposed across thebottom surface. The resin coating layers thus cover the top and bottomsurfaces of the resin impregnated fabric layer. Also shown is a secondresin impregnated layer 62, which also has a resin coating layer 54disposed across a top surface and a resin coating layer 56 disposedacross the bottom surface of the resin impregnated fabric layer 62.While fabric product 1 comprising two-layers of the resin layer-resinimpregnated fabric layer-resin layer groups is shown it should beappreciated that a fabric product could be comprises of any number ofresin impregnated fabric layers and resin layers.

The resulting fabric product 1 may contain one or more resin impregnatedfabric layers, 60 and 62 which are formed using a two-part castableurethane or other suitable resin system capable of providing resinpenetration to individual filaments of the fabric. As an example,traditional two part castable urethane systems use a resin to curativestoichiometry range of 85 to 110% theory. The curative stoichiometryrefers to the ratio of chemical components and the ratio of reaction toeach of the components. Typical stoichiometry are in the 95% range. Afabric product produced using a resin system 85-110% stoichiometry rangedid not achieve the desired performance.

Traditionally, stoichiometry range levels below 85% have not been used,with the belief that an unusable polymer would result. However, by usinga 75% stoichiometry range level, the resulting composite (also referredto as an impregnation compound) yielded both high shear strength underhigh seam loads and uniform strength throughout the composite. Thisresult is in direct contrast to prior experience and recommendations ofresin manufacturers.

In a particular embodiment the impregnation compound includes threecomponents: a urethane pre-polymer, a co-reactant curative, and adiluent, such as toluene. The impregnation compound in this embodimentis formulated as follows:

Polyurethane Specialties Pre-Polymer PCA 6-3 100.0 Parts by weightUniroyal Chemical Caytur 31 curative  26.1 Parts by weightThe mixture is then diluted to 75% total solids with toluene. The ratioof curative to pre-polymer is derived from the formula:

${\frac{6.34 \times 0.75 \times 230}{42} = {{parts}\mspace{14mu} {by}\mspace{14mu} {weight}\mspace{14mu} {of}\mspace{14mu} {curative}\mspace{14mu} {per}\mspace{14mu} 100\mspace{14mu} {parts}\mspace{14mu} {of}\mspace{14mu} {pre}\text{-}{polymer}}}\mspace{20mu}$

where 6.34 is the isocyanate content of the pre-polymer, 0.75 is thedesired stoichiometry, 230 is the equivalent weight of the curative, and42 is the equivalent weight of the isocyanate.

As described above, traditional fabric structures apply resin basedcoatings to the fabric. When a fabric is coated with resin, the coatingis only in contact with the outer filaments of the fiber bundles and,effectively, the fiber/resin system is full of voids or air pocketswithin the fiber bundles. When an applied load encounters a void, theload cannot be transferred or carried. The propagation of the loadeffectively stops and a stress concentration develops that eventuallybecomes greater than the fiber/fabric strength. These coating techniquesdo not drive the resin into the interstitial sites of the fabric norinto the individual fiber bundles or fiber filaments of the fabric.

A flow chart of the presently disclosed method for applying theimpregnating compound into the fabric is depicted in FIG. 2. Therectangular elements are herein denoted “processing blocks” andrepresent instructions or groups of instructions.

The method of producing the fabric product is described in conjunctionwith the flow chart of FIG. 2. The method 100 begins at processing block110 wherein the base fabric is scoured to remove any lubricants appliedby the yarn manufacturer, or those lubricants applied by the weaver.These lubricants could interfere with the development of high integrityresin to the base fabric interface. After scouring, processing proceedswith processing block 120.

Processing block 120 recites treating the fabric with a polymericisocyanate to enhance adhesion of the impregnation compound to thefabric. Processing block 130 is performed next wherein the impregnationof the base fabric is performed. The impregnation of the fabric involvesthe continuous submersion of the fabric in a tank containing theimpregnation compound.

At processing block 140, after the fabric emerges from the tank, thefabric is squeezed by a set of nip rolls to further drive theimpregnation compound into the fabric fibers and to remove any excessimpregnation compound.

The fabric is then fed into a drying oven, as shown in processing block150. Preferably the drying oven is set at a temperature as required toremove diluents from the fabric-resin composite.

Processing block 170 is executed next wherein the resulting impregnatedfabric is post cured at a temperature as required to cure the resinsystem. Following processing block 170 the fabric is ready forsubsequent processing.

The resulting impregnated fabric is incorporated into the fabric productand is used to produce fabric structures. In the case of a radomeincorporating the present invention, the radome fabric can withstandworking loads as high as 880 pounds per inch width for 56 hours at atemperature of 35 degrees C. in a humid environment. This inventionallows the use of thermally welded seams to meet these demandingrequirements. Seams made from this material are able to withstand inexcess of 56 hours at 880 pounds per inch load, with high humidity, at35° C. using an overlap seam construction. This seam performance has notbeen achieved in other flexible composite applications

A high strength, high modulus structural fabric product and the methodof manufacturing the product have been described. The incorporation of aspecific fiber/fabric treatment coupled with resin impregnation andcoating processes produces a composite material. This composite materialcomprises high strength and modulus fibers embedded in and coupled to amatrix. The resulting fabric product is useable in the formation ofseamed structures which carry and distribute high-level loads underextreme environmental conditions.

Having described preferred embodiments of the invention it will nowbecome apparent to those of ordinary skill in the art that otherembodiments incorporating these concepts may be used. Accordingly, it issubmitted that that the invention should not be limited to the describedembodiments but rather should be limited only by the spirit and scope ofthe appended claims. All publications and references cited herein areexpressly incorporated herein by reference in their entirety.

1-22. (canceled)
 23. A radome comprising: a fabric layer having aplurality of high strength and high modulus fibers, the plurality offibers impregnated by an impregnation compound with the fabric layercoated on each side with the impregnation compound, where theimpregnation compound is derived from a mixture comprising: apre-polymer; a co-reactant curative; and a diluent solvating the mixtureof the pre-polymer and the curative, and wherein the impregnationcompound has a curative stoichiometry range of less than 85 percent, andwherein the high strength and high modulus fibers comprise at least oneof aramid fibers or aromatic polyester fibers.
 24. The radome of claim23, further comprising a thermally-welded seam, wherein thethermally-welded seam comprises the fabric product.
 25. The radome ofclaim 24 wherein the thermally-welded seam has a strength to withstandan applied force of 880 pounds per square inch for at least 56 hours.26. The radome of claim 23 wherein the aramid fibers comprise polyaramidpolyparaphenylene terephthalamide fibers.
 27. The radome of claim 23wherein the aromatic polyester fibers comprise polyester-polyarylatefibers.
 28. The radome of claim 23 wherein the impregnation compound hasa curative stoichiometry range of approximately 75 percent.
 29. Theradome of claim 28 wherein a ratio of the co-reactant to the pre-polymeris about 26.1 to 100.0.
 30. The radome of claim 29 wherein the ratio ofthe curative to the pre-polymer is derived from the formula:${{\frac{6.34 \times 0.75 \times 230}{42} = {{parts}\mspace{14mu} {by}\mspace{14mu} {weight}\mspace{14mu} {of}\mspace{14mu} {curative}\mspace{14mu} {per}\mspace{14mu} 100\mspace{14mu} {parts}\mspace{14mu} {of}\mspace{14mu} {pre}\text{-}{polymer}}},}\mspace{20mu}$where the pre-polymer comprises an isocyanate and where 6.34 is theisocyanate content of the pre-polymer, 0.75 is the desired stoichiometryof the mixture, 230 is the equivalent weight of the curative and 42 isthe equivalent weight of the isocyanate.
 31. The radome of claim 23wherein the pre-polymer comprises a urethane pre-polymer.
 32. The radomeof claim 23 wherein the diluent comprises a solvent.
 33. A radomecomprising: a thermally-welded seam comprising a fabric layer, thefabric layer having a plurality of high strength and high modulusfibers, the plurality of fibers impregnated by an impregnation compoundwith the fabric layer coated on each side with the impregnationcompound, where the impregnation compound is derived from a mixturecomprising: a pre-polymer comprising an isocyanate having an equivalentweight of 42, the pre-polymer having an isocyanate content of 6.34; aco-reactant curative having an equivalent weight of 230; and a diluentsolvating the mixture of the pre-polymer and the curative, and whereinthe impregnation compound has a curative stoichiometry range ofapproximately 75 percent, wherein the high strength and high modulusfibers comprise at least one of aramid fibers or aromatic polyesterfibers, wherein the ratio of the curative to the pre-polymer is derivedfrom the formula:${{\frac{6.34 \times 0.75 \times 230}{42} = {{parts}\mspace{14mu} {by}\mspace{14mu} {weight}\mspace{14mu} {of}\mspace{14mu} {curative}\mspace{14mu} {per}\mspace{14mu} 100\mspace{14mu} {parts}\mspace{14mu} {of}\mspace{14mu} {pre}\text{-}{polymer}}},}\mspace{20mu}$where 6.34 is the isocyanate content of the pre-polymer, 0.75 is thestoichiometry range of the mixture, 230 is the equivalent weight of thecurative and 42 is the equivalent weight of the isocyanate.
 34. Theradome of claim 33 wherein the pre-polymer comprises a urethanepre-polymer.
 35. The radome of claim 33 wherein the diluent comprises asolvent.
 36. The radome of claim 33 wherein the thermally-welded seamhas a strength to withstand an applied force of 880 pounds per squareinch for at least 56 hours.
 37. The radome of claim 33 wherein thearamid fibers comprise polyaramid polyparaphenylene terephthalamidefibers.
 38. The radome of claim 33 wherein the aromatic polyester fiberscomprise polyester-polyarylate fibers.
 39. A radome comprising a fabricproduct comprising at least one resin fabric piece, the resin fabricpiece comprising: a resin impregnated fabric layer comprising highstrength and high modulus fibers and having a resin impregnated therein;a first resin layer having a resin disposed on a first side of the resinimpregnated fabric layer; and a second resin layer having a resindisposed on a second side of the resin impregnated fabric layer, whereinthe resin is derived from a mixture comprising: a pre-polymer comprisesan isocyanate having an equivalent weight of 42, the pre-polymer havingan isocyanate content of 6.34; a co-reactant curative having anequivalent weight of 230; and a diluent solvating the mixture of thepre-polymer and the curative, and wherein the impregnation compound hasa curative stoichiometry range of approximately 75 percent, wherein thehigh strength and high modulus fibers comprise at least one of aramidfibers or aromatic polyester fibers; wherein the resin has a ratio ofthe curative to the pre-polymer in accordance with the formula:${{\frac{6.34 \times 0.75 \times 230}{42} = {{parts}\mspace{14mu} {by}\mspace{14mu} {weight}\mspace{14mu} {of}\mspace{14mu} {curative}\mspace{14mu} {per}\mspace{14mu} 100\mspace{14mu} {parts}\mspace{14mu} {of}\mspace{14mu} {pre}\text{-}{polymer}}},}\mspace{20mu}$where the pre-polymer comprises an isocyanate and where 6.34 is theisocyanate content of the pre-polymer, 0.75 is the desired stoichiometryof the mixture, 230 is the equivalent weight of the curative and 42 isthe equivalent weight of the isocyanate.
 40. The radome of claim 39,further comprising a thermally-welded seam, wherein the thermally-weldedseam comprises the fabric product, and wherein the thermally-welded seamhas a strength to withstand an applied force of 880 pounds per squareinch for at least 56 hours.
 41. The radome of claim 39 wherein thearamid fibers comprise polyaramid polyparaphenylene terephthalamidefibers, and wherein the aromatic polyester fibers comprisepolyester-polyarylate fibers.
 42. The radome of claim 39, furthercomprising a second resin fabric piece disposed along a surface of thesecond resin layer, the second resin fabric piece being substantiallythe same as the first resin fabric piece.